1. Field of the Invention (Technical Field)
The present invention relates to in situ decontamination of soil, groundwater, and sludges containing ionic contaminants, especially groundwater containing anionic contaminants. In particular, the invention relates to the in situ immobilization of arsenic and chromate contaminants from water by injecting divalent metal cations into an aquifer.
2. Background of the Invention
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
For purposes of this disclosure, unless otherwise specified, the term xe2x80x9cmetal oxidesxe2x80x9d is intended to include both metal oxides and metal hydroxides. Similarly, for purposes of this disclosure, xe2x80x9carsenic contaminantsxe2x80x9d includes both arsenates and arsenites.
Arsenic contaminants are examples of anionic contaminants that may be present in groundwater or industrial wastewater as a result of natural as well as human mediated causes. The long-term availability of safe and affordable drinking water depends, in part, on availability of effective and economical treatment means for removing arsenic contaminants (as well as other anionic contaminants including chromate) from water. Successful treatment strategies, in turn, depend on not significantly altering the waters other characteristics (for example but not limited to, pH) in ways that would make it non-potable. Present treatment methods rely on pumping groundwater to the surface where it is processed above ground in a treatment facility. It would be cheaper if the contaminated groundwater could be treated in situ by injecting inexpensive treatment media directly into the ground, for example, into an aquifer.
Arsenic and other anionic contaminants likewise pose risks when present in fluids other than drinking water sources. For example, industrial wastewater streams often contain such contaminants, and their sources, typically sludges, require remediation and stabilization even when they are not considered to be directly associated with potable drinking water sources. Present art practices for sludge stabilization require expensive treatments, for example, lining of landfills.
Various sorbent materials for removing arsenic contaminants and other anionic contaminants from water have been used and developed previously. For example, certain metal oxide/hydroxide compounds, such as Al2O3 and Fe2O3, have been demonstrated to sorb anionic contaminants, including arsenic contaminants, from water. A drawback associated with use of such trivalent compounds alone is that, because they typically exhibit a point of zero charge from pH 7 to 9, the water to be treated may need to be acidified in order for these compounds to sorb anions to a significant degree. Thus, after treatment, in order to restore the potability of the treated water, further amendments must be added to bring the pH back up to an acceptable drinkable range. Similarly, tetravalent metal oxides such as SiO2 could be effective anion sorbents, however, their point of zero charge is typically around a pH of approximately 2, so extremely acidic conditions would be needed for tetravalent metal oxides to sorb anions. Additionally, these substances are considered likely to fall outside of the range of useful sorbents because of other chemical issues associated with operating at such low pH.
The divalent oxide MgO, likewise, has been shown chemically to sorb anions including arsenic in water. Although use of MgO does not necessitate driving the pH of water outside of the potable range (divalent metal oxides tend to exhibit a point of zero charge that is pH 10 or higher), the effectiveness of MgO as a sorbent for water decontamination, however, can be limited. This is due to its tendency to form carbonates in the presence of carbon in the water from natural (e.g., biological and atmospheric) sources. When this occurs, the carbonate species formed at the surface lack any significant electrostatic attraction for negatively-charged ions. Thus, the sorbency of the MgO can be short-lived absent taking steps to reverse the carbonate reaction and to restore the sorbent.
The sorbency methods just discussed rely on the electrostatic attraction between positively charged surface species and negatively charged contaminants. An altogether different mechanism that has been exploited to decontaminate water containing ionic contaminant species is ion exchange. Examples of ion exchange materials suitable for water decontamination include hydrotalcites (which exchange anions) and zeolites (which exchange cations). Although ion exchange materials have been shown to be effective without causing the types of problems associated with MgO (carbonate issues), ion exchange materials can be very expensive. Zeolites that allow for separations based on size are also used in some decontamination applications, but they do not sorb anionic species such as chromate and arsenic contaminants in water.
The need remains for improved decontamination approaches that are inexpensive, yet effective in removing anionic contaminants, including chromate, arsenates and arsenites. Further, there is a need for an approach that can be used in large-scale remediation in an inexpensive manner.
The present invention relates to an in situ process for treating ambient solid materials (e.g., soils, aquifer solids, sludges) by adding one or more divalent metal cations to the ambient solid material. The added divalent metal cations, such as Cu2+ or Zn2+, combine with metal oxide/hydroxides (e.g., ferric oxide/hydroxide or aluminum oxide/hydroxide) already present in the ambient solid material to form an effective sorbent material having a large number of positively-charged surface complexes that can bind and immobilize anionic contaminant species (e.g., arsenic or chromate) dissolved in any surrounding water. Divalent metal cations can be added, for example, by injecting an aqueous solution of CuSO4 into an aquifer contaminated with arsenic or chromate. Also, sludges can be stabilized against leaching of anionic contaminants through the addition of divalent metal cations. Also, an inexpensive sorbent material can be easily formed by mixing divalent metal cations with soil that has been removed from the ground.